The present invention relates to a radiation-sensitive recording material having a rough surface, which is particularly suitable for use in the preparation of planographic printing plates. The present invention also relates to a process for the production of this material, in which a rough covering layer is produced by spraying and drying a solution which has substantially the same composition as the recording layer.
It is known to provide light-sensitive planographic printing plates with a rough covering layer in order to accelerate evacuation of the copying frame during exposure according to the vacuum contact method. In printing plates having a smooth surface, evacuation of the space between light-sensitive layer and film original may be incomplete, since it is difficult to remove the air from the central portion, especially of large-size plates. The air pockets formed then lead to halation effects in the resulting copy. In these areas, the fine screen dots, in particular, which result are imprecise, incomplete or even in some cases not reproduced. These disadvantages can be obviated to a large extent by using printing plates having light-sensitive layers which have rough surfaces.
Rough surfaces have, for example, been produced by applying a covering layer containing pigment particles (German Offenlegungsschrift No. 2,512,043 corresponding to U.S. Pat. No. 4,168,979). Also, the surface of the light-sensitive layer or of an additional covering layer has been roughened by means of a coating roll having an uneven surface (German Offenlegungsschrift No. 2,533,156 corresponding to British Pat. No. 1,512,080). It is also known to incorporate pigment particles in the light-sensitive layer itself, which are thicker than the layer (German Offenlegungsschrift No. 2,926,236 corresponding to Canadian Pat. No. 1,148,014) or to evenly dust the layer surface with a solids powder which is anchored on the surface by heating (German Offenlegungsschrift No. 3,003,131 corresponding to U.S. Pat. No. 4,288,521).
All processes which use insoluble or sparingly soluble pigments, such as silica, have the disadvantage that these pigments later collect in the developer in the form of a deposit which may congest spray nozzles or cause other trouble.
It is also known to coat the light-sensitive layer with a rough covering layer which is completely soluble in the developer (German Offenlegungsschrift No. 2,606,793 corresponding to British Pat. No. 1,542,131). According to German Offenlegungsschrift No. 3,131,570 (corresponding to British Pat. No. 2,081,919), a covering layer is applied by spray coating from an aqueous solution or dispersion. The covering layer comprises a resin which is soluble either in water alone or at least in the developer solution employed.
A similar process is described in German Offenlegungsschrift No. 3,304,648 (corresponding to British Pat. No. 2,118,862). According to this process, a resin in aqueous solution or dispersion is applied by electrostatic spraying.
The covering layers obtained according to these processes have the disadvantage that they or the particles contained therein, respectively, adhere only relatively loosely to the light-sensitive layer. In fact, when water is used as a solvent or dispersing medium, the light-sensitive layer which is normally insoluble in water is not partly dissolved; therefore, firm anchoring cannot be obtained in the drying process. When a dispersion is used for coating, the dispersed particles are present on the layer surface in their original shape after drying and are not embedded in the layer. When an aqueous solution is used, particles which conform to the layer surface at their places of contact and, therefore, exhibit slightly better adherence, may be formed upon drying of this solution. Nevertheless, in this case also, the particles cannot be embedded in the layer, since the layer surface is not attacked by the spray solution. It is also a disadvantage of water-soluble resins that, during exposure, these resins may react with the photodecomposition products or, during the normally alkaline development, with diazo compounds present in the layer, to form a thin hydrophilic layer on the surface. This thin layer, if removed at all, is not entirely removed during developing. The consequence of this is an insufficient acceptance of ink in the image areas of the printing plate, at least in the beginning of the printing process, which results in a considerable number of waste sheets.
In addition, all covering layers which are soluble in the developer have the disadvantage of prematurely exhausting the developer, as their constituents are completely dissolved in the developing process.